1. Field of the Invention
This invention relates in general to data storage systems, and more particularly to a method and apparatus for adjusting digital filter taps based upon minimization of Viterbi margin counts.
2. Description of Related Art
Areal density specifies how many bits can be stored on a square inch of magnetic media. Areal density is defined as the number of bits/inch (hpi) of a track multiplied by the number of tracks/inch (tpi). To derive more storage capacity from storage systems, manufacturers must continue to push areal bits densities to new heights.
To achieve higher recording densities, new head designs, new media designs and improved read/write channels have played a pivotal role. As traditional technologies reached their performance limits, storage system designers looked to new approaches. One advance includes the use of partialxe2x80x94response, maximum likelihood (PRML) technology in the data channels.
However, noise, e.g., system generated noise or non-linear inter-symbol interference (ISI), degrades channel performance in PRML systems. For example, as higher densities are recorded on media, the magnetic-fluxer transitions between bits become crowded and create inter-symbol interference, i.e. one flux transition interferes with adjacent flux transitions. ISI has the adverse effect of shifting the phase and reducing the amplitude of two neighboring pulse due to superposition or non-linear bit shifts (NLB). Because PRML systems simple amplitude, the resultant phase-shift and reduction in amplitude causes improper sampling.
Equalization minimizes overall bit error rates that may be caused by noise. PRML channels employ finite-impulse-response shape. FIR filters may be optimized through error correction algorithms. Read channels may be designed with FIR filters so that errors from noise may be corrected up until the viterbi detector in a PRML channel.
In read channel equalization, the mean-squared error (MSE) method has been widely used to obtain the appropriate FIR tap weights or filter coefficients. The MSE method works generally well if the system is disturbed by random noise only. Because the objective of MSE equalization is used to minimize the mean-squared error of samples processed by the read channel, any localized defect is going to be xe2x80x9caveraged outxe2x80x9d in terms of its effect on the FIR taps. However, this kind of defect is going to cause severe error rate degradation. Therefore, the channel is usually not at the optimum operating point in terms of the best channel bit error rate.
Viterbi margin sample count at a certain level provides an indication of how many channel samples are separated from the decision boundary with that level. For example, a Viterbi margin count of 120 at level 9 means that there are 120 channel samples which are 9 least significant bits (LSBs) from the decision boundary. Another way that his can be expressed is if 9 LSB noise is added to any of these 120 samples, a channel bit error will occur. Under a real operating environment, some servo off-track occurs, which may produce multiple localized interference to the read operation. Accordingly, Viterbi margin counts directly reflects the potential bad channel samples.
It can also be seen that there is a need for a method and apparatus that adjusts FIR taps based upon Viterbi margin counts.
It can also be seen that there is a need for a method and apparatus that adjusts FIR taps to produce the best tap weights to produce the lowest bit error rate (BER) or lowest Viterbi margin count.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus that adjusts FIR taps based upon Viterbi margin counts.
The present invention solves the above-described problems by adjusting FIR taps to minimize Viterbi margin counts. Since Viterbi margin counts directly affect the channel BER, the method and apparatus according to the present invention produces FIR taps with a BER lower than using the MSE method. Therefore, the present invention provides better channel performance in terms of wider bandwidth and less hard errors.
A method in accordance with the principles of the present invention a) selects one of N tap pairs for adjustment, b) adjusts the tap pair up one count and down one count, c) determines whether adjusting the tap pair up or adjusting the tap pair down provides a lower Viterbi margin count from the Viterbi detector and d) selects the tap adjustment for the pair that provides the lower Viterbi margin count.
Other embodiments of a method in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the method further includes e) repeating b-d for the remaining tap pairs.
Another aspect of the present invention is that the method further includes f) repeating a-e N times to obtain an optimal Viterbi margin count.
Another aspect of the present invention is that the adjustment meets a predetermined constraint criteria.
Another embodiment of the present invention includes a read channel that includes a finite impulse response filter having a plurality of adjustable taps for shaping an input signal to obtain a desired output waveform, a Viterbi detector, coupled to the finite impulse response filter for determining a most-likely input sequence represented by a margin count and a processor, coupled to the Viterbi detector, for monitoring the margin count and adjusting the taps of the finite impulse response filter, wherein the processor a) selects one of N tap pairs for adjustment, b) adjusts the tap pair up one count and down one count, c) determines whether adjusting the tap pair up or adjusting the tap pair down provides a lower Viterbi margin count from the Viterbi detector and d) selects the tap adjustment for the pair that provides the lower Viterbi margin count.
Yet another embodiment of the present invention includes a data storage system that includes at least one disk for storing data in data thereon, a motor for rotating the at least one disk, an actuator arm assembly including a head for reading and writing data on each of the at least one disk and a data channel, operatively coupled to the head, for processing read and write signals to read and write data on the disk, the data channel further including a write channel and a read channel, wherein the read channel further includes a.finite impulse response filter having a plurality of adjustable taps for shaping an input signal to obtain a desired output waveform, a Viterbi detector, coupled to the finite impulse response filter for determining a most-likely input sequence represented by a margin count and a processor, coupled to the Viterbi detector, for monitoring the margin count and adjusting the taps of the finite impulse response filter, wherein the processor a) selects one of N tap pairs for adjustment, b) adjusts the tap pair up one count and down one count, c) determines whether adjusting the tap pair up or adjusting the tap pair down provides a lower Viterbi margin count from the Viterbi detector and d) selects the tap adjustment for the pair that provides the lower Viterbi margin count.
Another embodiment of the present invention includes an article of manufacture comprising a program storage medium readable by a computer, the medium tangibly embodying one or more programs of instructions executable by the computer to perform a method for adjusting taps in a FIR filter coupled to a Viterbi detector, the method comprising a) selects one of N tap pairs for adjustment, b) adjusts the tap pair up one count and down one count, c) determines whether adjusting the tap pair up or adjusting the tap pair down provides a lower Viterbi margin count from the Viterbi detector and d) selects the tap adjustment for the pair that provides the lower Viterbi margin count.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.